Research Technical Report

Intertidal Macroalgal Cover Modifies Pelvetiopsis Recruitment

ABSTRACT

One of the most conspicuous aspects of intertidal habitats along the coast of western North America is the distinct zonation pattern of biological organisms relative to tidal height. Ecologists initially attributed this pattern to biological interactions such as competition, however, more recently stress has emerged as an important factor in determining where different species live. In the high intertidal along the central California coast and on the rocky shores of the Monterey Bay National Marine Sanctuary, common stressors include high temperature during daytime low tides, high desiccation, and large waves on exposed coastlines during the winter storm swell season. This study addresses how intertidal algal canopies modify the survival of juvenile stages of the ubiquitous fucoid alga Pelvetiopsis limitata. Initial observations of Pelvetiopsis juveniles growing within the turf-like thallus of Endocladia muricata suggested a facilitative relationship between algal cover and algal recruitment. Permanent plots were established and algal cover was manipulated to include Endocladia only, Pelvetiopsis only, or complete removal of algal cover for one year. Pelvetiopsis recruits <1cm high in the different treatments were counted monthly. Results indicated that Pelvetiopsis recruitment was enhanced at intermediate levels of algal cover, and reduced at very low and high levels of cover. Furthermore, algal canopy species composition had little effect on Pelvetiopsis recruitment; this observation was further supported by the springtime appearance of Porphyra sp. in the plots and subsequent high numbers of Pelvetiopsis recruits. This relationship suggests that the potential for algal canopy cover to impact juvenile algal survival has more to do with the natural history of the canopy species (i.e. the timing of Porphyra recruitment) than with the composition of the canopy species (i.e. the structure of the Endocladia canopy vs. the Pelvetiopsis canopy). The relationship between algal canopy cover and recruitment success of intertidal organisms has important implications for modeling the population dynamics of dominant intertidal algae, and understanding patterns of species distribution and diversity.